Neurotransmitters and hormones are released by Ca2+triggered exocytosis, whereby a vesicle fuses with the plasma membrane of a cell and expels its content into the extracellular space. This project has focused on the fusion pore, the initial fluidic passage between the vesicle interior and the outside of a cell. Studies of the fusion pore have given us valuable insights into the mechanism of exocytosis and the roles of specific molecules in the regulation of membrane fusion. The previous funding cycle achieved many of the proposed specific aims, including identification of some of the fusion pore-forming proteins, the elucidation of key kinetic steps in the formation and dilation of fusion pores, and the elucidation of the roles of proteins in those steps. The present proposal follows up on these advances with studies to identify additional proteins in fusion pores and to compare the molecular composition of different classes of fusion pores. Furthermore, the scope of this project will be expanded to the study of fusion pores in synapses. Parallel studies of fusion pore manipulations between the PC12 cell model system and neurons from the brain will define how fusion pore function influences the speed of neurotransmitter release at synapses. Additional experiments are proposed to follow up on advances in understanding the role of a major protein complex (the SNARE complex) in driving membrane fusion.